Performing reliable underwater communication is relatively complicated. It is known that electromagnetic waves significantly attenuate when propagating through water. Practically the only frequency band that is used for underwater electromagnetic communication is the VLF (Very Low Frequency) range, in the range of up to 10 kHz. In this range, the transmission needs high power, and use of extremely long antennas are required. Therefore, such use is generally limited to submarine communications and cannot be applied for personal use. For short range underwater communication, conventional systems apply ultrasound acoustic waves, generally in the frequency range of 20 kHz-600 kHz. However, unfortunately, in the acoustic frequency range, the communication channel is limited to a relatively narrow bandwidth, which in turn limits the speed of the data transfer. The ability to reliably transfer data through water with acoustic waves is further harmed due to the non-linear speed of sound in water, different layers of water density, multipath of the propagation of the signal, fading and other environmental disturbances. Therefore, due to the abovementioned problems, many types of air communication systems are not applicable underwater.
Furthermore, most, if not all, of the currently existing systems for underwater communication employ analog modulation, a type of modulation which has been found inefficient for underwater network applications. For example, there exist underwater communication devices, such as the MKII by OTS company of Costa Mesa, Calif., which transmit vocal communication between divers, and are based on the transmission of ultrasound acoustic waves SSB modulated, generally in the range of 32-40 kHz. U.S. Pat. No. 4,563,758 discloses an underwater communicator device which permits acoustic communication between two divers by use of voice synthesizer. U.S. Pat. No. 5,523,982 also discloses a diver to diver communication apparatus which includes means for transferring messages between two divers, the messages determined by their audible tone or duration. The device further includes a flashing beacon for indicating the location of a diver in an emergency situation. CA 2,141,619 also discloses a device for transferring a message, amplitude modulated between two divers. The receiving device then emits an audible message that the diver carrying the receiving device can easily hear.
All the said devices and others enable communication between two divers being only in a relatively close range, generally in the range of less than 150 meters, and require a "line of sight" between the transmitting and receiving devices. They do not provide means for carrying out reliable underwater communication between any two or more divers or devices in a network, that may be located several kilometers away from one another, and lack a "line of sight".
Besides the abovementioned voice devices, there is no known reliable system which provides communication between divers, or more specifically, a paging system for transferring personal messages in a network of two or more divers.
It is therefore an object of the invention to provide a method for carrying out a reliable underwater communication.
It is another object of the invention to provide a low power underwater communication with a relatively long range.
It is still another object of the invention to provide a low power and reliable underwater communication between two devices lacking a "line of sight".
It is still another object of the invention to provide an underwater-dialing network in which communication can be carried out between any two or more members in the network.
It is still another object of the invention to provide an underwater paging network for transferring messages between divers.
It is still another object of the invention to provide a personal, low-power and lightweight device, capable of receiving, displaying, and transmitting messages to or from other users underwater.
It is still another object of the invention to provide means for transferring emergency messages to or from one or more divers.
It is still another object of the invention to provide underwater communication means which do not require full mask equipment.
Other purposes and advantages of the invention will become apparent when the description proceeds.